Physicists have long sought to understand the irreversibility of the world around them and assume that its appearance is the fundamental laws of physics, symmetrical over time. According to quantum mechanics, the ultimate irreversibility of time reversal in concept requires extremely complex and unimaginable situations that are not likely to occur naturally in nature. Physicists have previously pointed out that although time reversal is not possible exponentially in natural environments ̵

1; it is possible to design an algorithm to artificially reverse the arrival time arrow. a known or given state in an IBM quantum computer. However, this version of the reversed time arrow accepts only a known quantum state and is therefore compared with the quantum version of pressing rewind on the video to “reverse the timeline”.In a new report is currently published in *Communication physics*Physicists AV Lebedev and VM Vinokur, and colleagues in advanced materials, physics and engineering in the US and Russia, built on their previous work to develop an engineering approach to reverse engineering. reverse the evolution over time of an unknown arbitrary quantum state. Engineering work will open up new routes for common universal algorithms to take the temporal evolution of an arbitrary system over time. This work only outlines the mathematical process of inversion of time without experimentation.

**Time arrow and develop a time reversal protocol**

The time arrow stems from showing the direction of time in a singular way compared to the second law of thermodynamics, implying that the growth of entropy comes from the dissipation of the system’s energy. environment. Therefore, the scientists can consider energy loss to be related to the system’s entanglement with the environment. Previous work only focused on the quantum view of the time arrow and explored the effect of the Landau-Neumann-Wigner hypothesis to quantify the complexity of time arrow reversal on IBM quantum computers. . In this study, the scientists proposed to use a thermodynamic tank at finite temperature to form a high entropy random tank to heat a certain quantum system and experimentally increase turbulence. heat or entropy in the system. However, in the experiment, the IBM computer does not support the heat dissipation process, which is the first step in the currently proposed cycle.

In theory, the presence of a sudden heat tank made it possible to prepare thermal states at high temperatures of a sub-quantum system (alternatively) elsewhere, regulated by the same Hamilton ( an operator that corresponds to the sum of the kinetic and potential energies for all the particles in the system). This allowed Lebedev and Vinokur to invent the mathematical mathematics of retrograde evolution to reverse the dynamics of time in a given quantum system.

**Universal process and backend system**

The team determined the universal time reversal of an unknown quantum state using the density matrix of a quantum system (mixed state); to describe the evolutionary reversal of the time system to return to the original state. The quantum state of the new system may remain unknown while performing a time reversal arrow. Contrary to the previous time reversal protocol of a known quantum state, the initial state is not necessarily a completely unrelated state and can be maintained in a mixed state and correlated with interactions in the past with the environment. The team noted that the time reversal complexity was reduced for the high entropy mixed state in the system.

Lebedev et al. has relied on the inversion procedure detailed earlier by S. Lloyd, Mohseni and Rebentrost (LMR procedure) to construct or map the initial density matrix. The LMR procedure looked at the matching arrangement of the system in question and a plan to perform the reversible computation. The experimental system will be equipped with a thermodynamic bath to heat the ancilla and provide the desired state for reverse evolution. The hotter the system, the more chaotic it will become. By using the heat tank to expose the auxiliary system to extremely high temperatures, Lebedev et al. The paradox is for the purpose of experimentally observing the cold and ordered past of the primary system using the LMR formula. The authors reason that a universal time inversion algorithm can run a reverse calculation, with no specific quantum state to rewind, as long as the algorithm facilitates the return of time. its original point.

**Computational complexity of the time-reversal procedure**

The work only outlines mathematical analyzes of the inversion of time, but does not specify experimental implementations. While performing a time reversal, the proposed system continues to maintain the forward evolution tuned by Hamilton itself. The computational complexity of the inversion of time for an unknown quantum state is proportional to the square of the Hilbert dimension of the system (an abstract vector space). To do this in practice, the experimental system would require a natural system that evolved under an unknown Hamilton along with a thermochemical process, which the quantum computer does not support, is paired with Universal quantum gate to reverse time. Hence, a real implementation of this work would require an upgrade to existing quantum computers to fulfill the stated requirements.

**Existing quantum chip design upgrade roadmap**

Lebedev et al. thus, it aims to upgrade the existing design of quantum chips to achieve a set of interactive qubits (quantum bits) that can be thermally required in high-temperature environments. To do this, the superconducting qubits can be paired with a transmission line where high-temperature radiation will be provided to put the qubits in a high-temperature state. They will then request a second qubit set that can store a quantum state similar to the original qubit set. When the initial set of qubits is then experimentally heated to perform common LMR evolution, subsequent qubits will be able to undergo time-reversing dynamics under the same Hamilton to reach the initial state. head. If implemented correctly, the proposed mechanism will also facilitate error correction for an upgraded quantum computer to validate its correct function. Lebedev et al. visualize process execution on emerging computers with on-demand thermochemical qubits.

In this way, Lebedev and Vinokur demonstrated the time reversal process of an unknown mixed quantum state. The process relies on the implementation of the LMR protocol and the existence of an ancilla system, whose dynamics can be regulated by Hamilton just like Hamilton’s reverse system. To perform the reverse procedure, the LMR protocol will need to be applied sequentially to the general state of the system and ancilla, prepared in the thermal state. The work has developed a formula to highlight the number of cycles that need to be repeated to reverse the state of a given system to a previous state in the past. This number will depend on the complexity of the system and the amount of time it is expected to pass. When implementing the time reversal protocol, the operating speed of the LMR process must be high enough to pass the reversed system time transition.

The thermal turbulence brings the quantum system back to its unknown past

**More information:**

AV Lebedev et al. Time reversal of an unknown quantum state,

*Communication physics*(Year 2020). DOI: 10.1038 / s42005-020-00396-0

Seth Lloyd et al. Analyze main quantum components, *Natural physics* (2014). DOI: 10.1038 / nphys3029

Gonzalo Manzano et al. Theorems of quantum vibrations for arbitrary media: Production of adiabatic and non-adiabatic entropy, *Physical evaluation X* (2018). DOI: 10.1103 / PhysRevX.8.031037

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**Quote**: Time reversal of an unknown quantum state (2020, August 10) retrieved August 11, 2020 from https://phys.org/news/2020-08-time-reversal- unknown-quantum-state.html

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